For more information on the Columbia River Highway features discussed in the State Trail Plans:

• Mitchell Point

Jeanette B. Kloos, Historic Columbia River Highway Master Plan (2006)

j) Mitchell Point

Old mile post 61.1–61.3

Existing Conditions – Interpretive information at Mitchell Point tells visitors about the destroyed “Tunnel of Many Vistas.” The original tunnel location is a ledge, being used as a rockfall catch area.

Vision – Construct HCRH State Trail at Mitchell Point. Recreating the tunnel is proposed, because this proposal would be more likely to meet the visual requirements, as seen from Interstate 84. An additional, lower cost project is proposed to enhance the viewpoint, where the interpretive sign is located, with landscaping.

The Historic Columbia River Highway: Mile Post 2016 Reconnection Projects (March 2009)

Segment 8 - Mitchell Point Tunnel

At this location, a tunnel originally connected the two intact historic highway segments on either side of Mitchell Point. The “Tunnel of Many Vistas” was 390 feet long and had five adits, or “windows,” with magnificent views of the scenic Columbia River. The tunnel was 18 feet wide and 19 feet high at the crown. A viaduct was used to access the tunnel’s west portal, and a cut into the hillside accessed the west end of this span. The state closed the old highway route through the Mitchell Point Tunnel in 1953 because of increased vehicle size and rockfall hazards. The tunnel and much of the approach route to the west, including the viaduct, were removed when the state removed the hillside to make room for full build-out of Interstate 84. The only remnants of the tunnel are narrow ledges at the west and east approaches. These benched areas are located about 200 feet above Interstate 84 and currently used as rockfall catchment areas in conjunction with other catchments adjacent to Interstate 84 below.

Reconstruction of Mitchell Point Tunnel would not only provide a way through Mitchell Point but also provide future trail users and visitors a unique way to experience the historic highway and Columbia River Gorge. In spring 2008, the Friends of the Historic Columbia River Highway contracted with GRI, geotechnical engineers, to prepare a geotechnical feasibility study for a new tunnel through Mitchell Point.

Their report envisions a tunnel that connects the two old highway segments east and west of Mitchell Point near the original grade of the historic highway. With a tunnel width and height of 18 feet and length of about 1,200 feet. Tunnel report includes three to five adits (windows) from the main tunnel to the cliff face to the north to provide viewing areas and allow for natural light. The west portal is located at the existing rock outcrop near the northeast corner of the parking area. The east portal requires a cut in soil and decomposed rock before tunneling can begin.

Trail Segment Highlights:

• A tunnel would become a landmark, a monument, and a tourist attraction. This is the most cost effective way around Mitchell Point (Photo 8C).

• OPRD recently received funding through Forest Highway Enhancement Funds to redesign and enhance the parking lot at Mitchell Point (Site Plan A and Photo 8A).

• Most visually subordinate of the all options to get around Mitchell Point.

• Site is accessible to eastbound I-84 motorists.

• Incredible Columbia River Gorge views (Photo 8B)

August 5, 2020

The restoration of the trailhead and viewpoint was completed years ago. Visit the Mitchell Point Viewpoint (MP 61) page for before and after photos of the restoration.

The Historic Columbia River Highway: State Trail Plan - Wyeth to Hood River (Winter 2010)

• Section 526+50 – 444+44: The next portion of the Trail follows an existing section of Historic Highway passing I-84 Exit 58 and running through the Mitchell Point West Trailhead. This section of trail also provides access to the Wygant Trail. New overlooks are proposed north of the parking area to allow better access to viewing the original alignment of the Historic Highway. The existing Historic Highway alignment will be preserved through this area and will be better defined by the redesigned parking area.

Mitchell Point West Trailhead Description:

• Existing large asphalt parking area

• No definition of existing HCRH

• Existing overlook with interpretive sign

• Chain link fences to protect visitors from fall

• Existing rockfall into existing parking area

• Existing restroom

• Site has various remnants of past commercial development

• Invasive species and remnant ornamental landscape plantings

Trailhead Amenities:

• New Tunnel through Mitchell Point

• Reconfigure parking area to provide for better traffic flow and turnaround

• Maximum 25 car parking area at existing parking area

• Provide bike and hike trip staging area

• New overlooks with interpretive signage

• Improve trail access through the site

• Provide trail information, directional signage

Mitchell Point West Trailhead:

Site Preparation $96,000.00

Grading and Drainage $31,000.00

Wall Construction $78,000.00

Paving $120,000.00

Rock Fall Protection $200,000.00

Misc. Trail Improvements $68,000.00

Landscape Improvements $27,000.00

Subtotal $665,000.00

Engineering & Permits (20%) $133,000.00

Construction Engineering (15%) $100,000.00

Contingency $179,000.00

Project Costs (2010) $1,077,000.00

Project Costs (2014) $1,270,000.00

The Historic Columbia River Highway: State Trail Plan - Wyeth to Hood River (Winter 2010)

• Section 444+44 – 457+00: A new 1200 foot tunnel will recreate the passage through Mitchell Point. The tunnel will feature windows carved through its side, at the location of the former viaduct, to allow views of the Columbia River and recall the original Mitchell Point Tunnel, “Tunnel of Many Vistas” that had five windows carved in its side. This Section ends at the East Portal of the Tunnel.

Trail Segment Highlights:

• 1,200 foot tunnel

• Viewpoints at Mitchell Point West

• Viewpoint at center of viaduct

• Viewpoint at Mitchell Point East

• Quarry floor restoration area

• Peregrine Falcon habitat

Issues for Further Study:

• Affects of Tunnel Construction on Mitchell Point

• Consultation with Native American Tribes on impacts to this significant and culturally important landmark

• Effect on Peregrine Falcon Habitat

• Rockfall issues at the Trailhead

• Additional archaeological investigation of HCRH resources that may exist at the Trailhead location

• Potential location of a Trail Host Site at Mitchell Point West Trailhead

Segment F Costs:

Site Preparation $9,000.00

Grading and Drainage $41,000.00

Wall Construction $91,000.00

Trail Construction $140,000.00

Rock Fall Protection $200,000.00

Tunnel $6,180,000.00

Landscape Improvements $170,000.00

Subtotal $6,841,000.00

Engineering & Permits (20%) $1,368,000.00

Construction Engineering (15%) $1,026,000.00

Contingency $1,847,000.00

Project Costs (2010) $11,082,000.00

Project Costs (2014) $13,077,000.00

Funding (As of May 2013)

Environmental: Complete

Engineering: $2 Million Needed

Construction: $11.1 Million Needed

Total Funds Needed: $13.1 Million

ODOT, FINAL REPORT: MITCHELL POINT FEASIBILITY & COST STUDY (June 30, 2015)

4.1 HISTORIC TUNNEL

A National Park Service HAER record of the Mitchell Point Tunnel provides the following description of historic infrastructure (Brooks, 1995)

In 1915, John Arthur Elliott designed and engineered one of the most difficult, and beautiful, sections of the Historic Columbia River Highway – Mitchell Point. Because the OWRR&N (Oregon‐Washington Railroad & Navigation Co.) occupied the water‐level route, Elliott built a viaduct and tunneled through Lower Mitchell Point (Mitchell Spur) to achieve the most direct and economical route possible. The new section replaced a tortuous wagon route, and reduced the journey around Mitchell Point by almost three miles.

Mitchell Point Viaduct spanned a natural break in the cliffs of Lower Mitchell. Built entirely of reinforced concrete, the viaduct rested on six sets of columns with footings in a steep talus slope. A series of six 32 foot slab spans carried the viaduct 192 feet to the west portal of the tunnel. From curb‐to‐curb, the roadway measured 20 feet, and paving brick decorated the railing posts. Standifer‐Clarkson, of Portland, completed the viaduct in 1915 at a cost of $9,201.

Mitchell Point Tunnel was 390 feet long, 18 feet wide and 19 feet high at the crown. It was one of the first highway tunnels to use adits, or “windows”, to light the interior and allow views of a scenic landscape. Early motorists stopped their cars inside the “Tunnel of Many Vistas” to gaze out one of the five windows, enjoy the view, or walk a short trail leading from the fifth window.

A 10° curve about halfway through the tunnel allowed motorists a view of the landscape from moving cars. The tunnel cost $16,221, and was completed by Standifer‐Clarkson in 1915.

By 1953, larger vehicles had rendered the tunnel obsolete, while unstable rock had made it dangerous. As a result, the route was relocated to water‐level, the tunnel was filled, and the Mitchell Point section of the Historic Columbia River Highway abandoned. In 1966, the widening of Interstate 84 required that the cliff be scaled back. As a result, the tunnel and viaduct were destroyed.

4.2 SITE CONDITIONS

Mitchell Point is a prominent cape extending to the Columbia River within the Columbia River Gorge. It consists of a lower peak, known as Mitchell Spur, which is close to the river and extends to an elevation of approximately 500 feet. A saddle is located between the Mitchell Spur and the higher peak, Mitchell Point, which climbs to the south with a peak elevation of over 1,000 feet. The saddle between the two peaks has a minimum elevation of about 400 feet.

North facing slopes are generally steeper than south facing slopes, likely due to the rock dipping toward the southeast. North facing rock cuts approach vertical for heights up to 100 feet in the vicinity, primarily below the existing bench. Above the bench rock cuts and slopes generally range from 4V:1H to 1V:1H and may be several hundred feet in height. South facing slopes are generally vegetated and not as steep. Drainage occurs to the east and west from Mitchell Point and then north toward the Columbia River.

The Friends of the Historic Columbia River Highway have commissioned past efforts to study a new alignment which have included preliminary geotechnical reconnaissance, details are provided in GRI (2008).

4.3 SITE VICINITY GEOLOGY

Several members of the Miocene aged Columbia River Basalt Group are mapped in the project vicinity. Mitchell Spur is composed of Grande Ronde Basalt with the Frenchman Springs Member of the Wanapum Basalt, Troutdale Formation, and the Pomona Member of the Saddle Mountains Basalt mapped above at Mitchell Point (Anderson, 1980; Korosec, 1987), as shown in Figure 1. Talus slopes and Troutdale Formation clasts were observed during the site visit on the eastern flank of Mitchell Point. The saddle between Mitchell Spur and Mitchell Point formed the southern branch of the Bridal Veil channel of the ancestral Columbia River (the northern branch was destroyed when the present day Columbia River began incising the Gorge) (Tolan et al., 1984). It is assumed Grand Ronde Basalt is the bedrock material that would be encountered along any chosen alignment.

The best readily available information and description of the character and structure of the Grand Ronde basalt in the immediate vicinity of Mitchell Point and as it relates to possible tunnel alignments through the point can be found in the geotechnical evaluation performed by GRI in their 2008 study of a tunnel option through Mitchell Point. Selected excerpts on the site conditions from the GRI report that are particularly relevant to judging the general tunnel constructability and estimating ground support requirements are the following (GRI 2008):

• “The rock is typically hard (R4) and fresh to slightly weathered. Four basalt flows are visible in the outcrops located parallel Interstate I‐84 and adjacent the parking lot at the west end of the project.
• …. the rock dips down to the southeast generally at about 26 degrees with a strike of about 30 degrees.
• Fracture patterns and spacing vary depending on the location within each basalt flow. In general, columnar fracture patterns are well defined within approximately the bottom quarter to third of each flow. Fractures within the columnar zones are typically moderately close to widely spaced with about 1 to 4 feet between fractures.
• Fractures in the upper two‐thirds of each basalt flow appear randomly oriented and are very close to close, typically about 1 to 6 in. The randomly oriented fractures are interconnected, but do not appear to form persistent linear fractures through the rock mass.
• A weathered contact between two basalt flows will likely be encountered about 400 feet from the west portal. … The rock below this contact is predominantly decomposed for a thickness of about 15 ft. “

ODOT, FINAL REPORT: MITCHELL POINT FEASIBILITY & COST STUDY (June 30, 2015)

5.1 ALIGNMENT 1 – TUNNEL OPTION

5.1.1 OVERVIEW

This option depicts the approximate alignment of the tunnel described in the GRI study (GRI, 2008). The west portal of the tunnel begins about 80 feet north of the existing parking lot. The alignment then bears eastward in a straight line toward approximately the center of the shallow gully in the north face of the cliffs. Side adits allowing for views of the river may be constructed at this location. The tunnel then bears in a straight line through the next segment of the cliffs to the east, ending on the far eastern side of the point for a total length of approximately 1,225 feet.

5.1.2 STRUCTURAL/TUNNEL DISCUSSION

A drill and blast mined horseshoe shaped tunnel section is anticipated for this option. A suitably sized roadheader may be able to mine the tunnel, and a hoe ram could be used to conduct limited excavation. However, given the observed rock quality and fracture patterns, it is anticipated drill and blast methods would be a more economical means to excavate the tunnel.

For purposes of assessing tunnel options and estimating their costs we have developed a trial tunnel cross section (see Detail 1) and conceptual ground support schemes (Details 2‐5) for a typical portal in rock plus three ground support categories that together provide a representative range of what we consider to be reasonable, cost effective and readily constructible support methods to address the range of anticipated ground conditions as inferred from our site visit and the limited available geologic information. Table 2 summarizes the main attributes and intended setting for reach ground support category.

All categories assume a functional but simple shotcrete final lining. While not watertight without a membrane system, the shotcrete will serve to deflect groundwater. Where active seepage is encountered localized treatment using geocomposite drain materials can be used to drain the groundwater to the invert and allow the shotcrete to then be placed.

Considering the relatively dry conditions a dedicated subdrain pipe system is not considered necessary. Gutters at the base of each sidewall combined with a slight crown in the pavement are considered sufficient to keep collected flows off of the traveled portions of the invert pavement and efficiently deliver it to the downhill portal.

The rock formation may provide adequate support to allow portions of the sidewall to be exposed for aesthetics. However, it is anticipated the majority of the tunnel will require shotcrete, especially the crown, to protect users against possible rockfall.

5.2 ALIGNMENT 2 – SHELF & BRIDGE/TUNNEL OPTION

5.2.1 OVERVIEW

Alignment 2 intends to take advantage of the remnant shelf along the basalt face from the original HCRH construction. Alignment 2 is further subdivided into three different options using a combination of shelf structure, bridge structure and tunnel. Given the rockfall hazard and intended use of the trail, Alignment 2 concepts consider that all exposed sections of the trail will be protected through some means of rockfall mitigation. Mitigation concepts considered include a rockfall shelter, similar to the Mosier Twin Tunnels east of Mitchell Point, or mesh and cable net slope protection, which could also include rock bolting and shotcrete as necessary.

Alignment 2 begins at the existing trail terminus and continues coincident with the existing historic highway past the parking lot. The trail continues along the river‐facing shelf, making use of the existing rockfall catchment area.

Where the shelf ends and the cliff face begins to curve back toward the south, Alignment 2 splits into Options 2A and 2B. Option 2A continues east over a proposed bridge structure for approximately 300 feet before touching down again on the eastern, existing shelf. Option 2B rounds the inside of the cliff face within a curved tunnel for approximately 380 feet and ends at the same point as the bridge abutment. The tunnel alignment adds about 80 feet to the total length compared to Option 2A. Both 2A and 2B continue easterly along the shelf following the same alignment to the terminus chosen for this study.

Option 2C follows the same alignment as the tunnel in Option 2B but breaks off from the curve at approximately the halfway point of the 2B alignment and continues east through the rock in a tunnel for a total length of approximately 825 feet. This alignment daylights on the far eastern side of Mitchell Point. The total length of alignment 2C is approximately the same as 2A.

5.2.2 STRUCTURAL/TUNNEL DISCUSSION

Option 2A‐ Shelf and Bridge:

Option 2A utilizes the shelf of the original HCRH alignment to the fullest extent. A gouge or chute, forming a chasm in the rock face exists where the original highway’s bridge structure was located and later was removed. In this option, a bridge is used to cross the chasm. As mentioned above, rockfall mitigation in the form of rock shelters or mesh and cable net slope protection are considered for both the eastern and western trail approaches to the bridge.

Bridge:

Option 2A includes adding a bridge across the chasm in the shelf. The gap that the bridge would need to span is approximately 300 feet in length. This estimated bridge length of 300 feet accounts for an assumed weathered rock zone near the existing edges of the chasm. Since the original Columbia River Highway alignment was along the shelf, it is reasonable to assume that the shelf consist of competent material suitable for a bridge foundation.

Both single and multi‐span bridge types are feasible for spanning the 300 feet. For a conventional type of bridge (deck and girder), this span length would require one or two intermediate bents. With the shelf elevation approximately 75 feet above the elevation of I‐84, these long intermediate bents, would be fairly expensive.

It is feasible to span the 300 foot gap with a single span bridge and avoid intermediate bents. At the approximate span length of 300 feet, the family of feasible bridge types includes a deck arch, tied arch, cable stayed and suspension bridges.

Examples of deck arch style bridges include the Shepperd’s Dell Bridge and the 1915 Moffett Creek Bridge (see Figure 5 – Moffett Creek Bridge, circa 1920).

Local examples of the other bridge types mentioned include:

• The Fremont Bridge is a tied arch bridge.
• TriMet’s new Tilicum Crossing Bridge is a cable stayed bridge
• St. John’s Bridge is a suspension bridge.

The usable trail width is 16 feet. Including bridge railing outboard of the trail, the overall bridge deck width is considered to be 19 feet wide.

The adjacent weathered rock face creates a rockfall hazard for pedestrians using the trail as well as the structural integrity of the bridge. A geotechnical/geologic investigation of the rockfall, as well as refinement of the trail alignment would be needed to determine the risk to the bridge related to rockfall. It is recommended to investigate mitigation measures such as locating the bridge’s alignment outside of the rockfall zone or using mesh and cable net slope protection.

Shelf Widening:

From site visit observations, the width of the existing shelf, in many locations, is less than the required 16 feet of usable trail width. Possible strategies to provide a usable trail width of 16 feet include:

• Cantilevered decking over the shelf edge,
• Scaling the rock face to achieve the 16 foot trail width and mesh and cable net for slope protection or
• Rock shelter with localized rock scaling or excavation to achieve the 16 foot trail width.

Cantilevered Decking: In this concept, concrete deck panels would be anchored vertically into the existing shelf and cantilever over the edge of the shelf edge. With the rockfall hazard still present, mesh and cable net slope protection is added to mitigate the hazard.

Scaling and Mesh and Cable Net: In this concept, the rock face is scaled to remove loose and weathered rock. Mesh and cable netting is installed for slope protection. Some rock bolting and shotcrete may also be needed to complete the rockfall mitigation. Mesh and cable net slope protection is discussed in further detail in ODOT’s 2005, I‐84 Corridor Strategy.

Rock Shelter and Localized Scaling: In this concept, a rock shelter is utilized to mitigate the rockfall hazard. The roof edge of the shelter follows the contour of the rock facing. There will be some localized areas, perhaps as wide as 6 to 7 feet, where rock scaling will need to occur in order to achieve a usable trail width of 16 feet.

Scaling is a common practice to remove weathered, loose, and hanging material from a rock slope in a relatively controlled, proactive fashion. The work can proceed manually using picks and prybars, or mechanically using hoe rams. Specialized techniques are also used depending on rock type including hydro‐scaling and expansive (non‐explosive) compounds. Workers usually gain access to the slope via crane‐supported baskets or suspended on ropes. Scaling is typically specified after controlled blasting of a new rock cut, or as part of a periodic maintenance program on existing slopes.

Option 2B‐ Shelf and Short Tunnel:

Option 2B is similar to Option 2A except that a curved tunnel structure is used to bypass the chasm instead of a bridge.

Short Tunnel:

The curved tunnel hugs the rock face and could include one or more adits for viewing ports. See discussion in Section 5.1.2, except that the ground support for the east portal would mimic that for the west (that is, lattice girders are not anticipated).

Shelf Widening:

From site visit observations, the width of the existing shelf, in many locations, is less than the required 16 feet of usable trail width.

Possible strategies to provide a usable trail width of 16 feet include:

• Cantilevered decking over the shelf edge,

• Scaling the rock face to achieve the 16 foot trail width and mesh and cable net for slope protection or

• Rock shelter with localized rock scaling to achieve the 16 foot trail width.

Cantilevered Decking: In this concept, concrete deck panels would be anchored vertically into the existing shelf and cantilever over the edge of the shelf edge. With the rockfall hazard still present, mesh and cable net slope protection is added to mitigate the hazard.

Scaling and Mesh and Cable Net: In this concept, the rock face is scaled to remove loose and weathered rock. Mesh and cable netting is installed for slope protection. Some rock bolting and shotcrete may also be needed to complete the rockfall mitigation. Mesh and cable net slope protection is discussed in further detail in ODOT’s 2005, I‐84 Corridor Strategy.

Rock Shelter and Localized Scaling: In this concept, a rock shelter is utilized to mitigate the rockfall hazard. The roof edge of the shelter follows the contour of the rock facing. There will be some localized areas, perhaps as wide as 6 to 7 feet, where rock scaling will need to occur in order to achieve a usable trail width of 16 feet.

Option 2C‐ Shelf and Intermediate Tunnel:

Option 2C’s western approach utilizes the original HCRH alignment for the trail and then constructs an intermediate length tunnel along a portion of the rock face and then on to the east, along the Option 1 alignment.

Intermediate Tunnel:

For a discussion of the Intermediate Tunnel, see discussion in Section 5.1.2.

Shelf Widening:

From site visit observations, the width of the existing shelf, in many locations, is less than the required 16 feet of usable trail width.

Possible strategies to provide a usable trail width of 16 feet include:

• Cantilevered decking over the shelf edge,

• Scaling the rock face to achieve the 16 foot trail width and mesh and cable net for slope protection or

• Rock shelter with localized rock scaling to achieve the 16 foot trail width.

Cantilevered Decking: In this concept, concrete deck panels would be anchored vertically into the existing shelf and cantilever over the edge of the shelf edge. With the rockfall hazard still present, mesh and cable net slope protection is added to mitigate the hazard.

5.3 ALIGNMENT 3 – OVERLAND OPTION

5.3.1 OVERVIEW

Alignment 3 consists of overland options constructed through the saddle between Mitchell Point and Mitchell Spur. Alignment 3 options consist heavily on switchback trails instead of tunnels or structures.

Option 3A is the overland option with the least amount of earthwork. Alignment geometry is laid out to hug existing contours while meeting existing design guidance to the extent feasible. Due to the steepness of the slope being traversed, in many areas a shorter radius curve of 20’ was used to avoid excessively tall walls and extra earthwork at these locations. The path location and alignment geometry is highly subjective, meaning each designer may decide to lay it out differently. For this study the primary goal is to show the approximate length and impact of a 16’ wide overland path if an average of 5% grade is used from the bottom up to the saddle, and back down. A detailed design of this path could result in the use of 8%‐12% grade runs as allowed by design guidance. Additionally, consideration of a narrower path could potentially reduce earthwork or the need for walls in some locations.

The switchback path approaching and exiting the saddle is approximately 5,000 feet long on each side to accommodate about 250 feet of grade change at 5% grade. Landing areas of 2% grade at each switchback are needed and could increase the required path length. This would require further study and layout during subsequent phases of the project should the overland options be preserved. Retaining structures with railing will be required at the switchbacks because the surface colluvium (talus) would not likely support a cut slope at these locations. Earthwork quantities for the Alignment 3 options were estimated using average cut and fill amounts for typical path locations on the slope. Exact modeling using design software was not performed. On the east side of the saddle, switchbacks are used to touch down at approximately the same location as the other alignment options.

Option 3B features a through‐cut near the saddle as a means of reducing the length of the approach trails. For this specific case, a 50 foot high cut was considered. The cut is based on slopes of 0.5H to 1V.

Rock bedding and fracturing may require different slope inclinations on one or both cut faces. By cutting out 50 feet of height, the path length can be reduced by approximately 1,600 feet. The alignment of Option 3B leading up to and away from the saddle is approximately the same as 3A up to the 350 foot elevation mark. The total alignment length of option 3B with the cut is approximately 7,500 feet versus the length of 9,100 feet for option 3A.

A short tunnel near the saddle was briefly considered as a means similar to a cut at reducing the trail length. A tunnel feature was dropped from further evaluation because of the likely presence of poor ground conditions that would make portal and tunnel excavation difficult. The two aerial views of Mitchell Point below (Figure 9 and Figure 10) show the saddle to be comprised of a less erosion and weathering‐resistant unit of the Grand Ronde Basalt than the prominent underlying outcrop that makes up Mitchell Spur and through which the original tunnel was constructed. The northeast strike and southsouthwest dip of the lower flow unit suggest:

• Any tunnel alignment through the spur that trends more easterly than the strike of the lower flow unit is likely to encounter more of the overlying weaker rock unit that makes up the saddle and east facing talus slopes
• A tunneled variation to the surface trench in Option 3B would have to be much deeper and trend in a more northerly direction to remain in the same flow unit as the original tunnel was excavated and as Options 1 and 2 would encounter.

It is conceivable that the number of switchbacks (and associated walls) could be reduced by extending the Option 3 alignments to the east and west beyond the areas shown. However, these were not considered further due to the presence of talus slopes and outcrops that would make such routes difficult to achieve.

5.3.2 STRUCTURAL DISCUSSION

Option 3A‐ Overland Trail: Option 3A connects west to east by using an overland trail. Due to the steepness of the terrain, trail design requirements call for many switchbacks, as the trail carves its way up the hillside.

The steep terrain also requires retaining walls as the trail traverses switchbacks. Generally, feasible retaining walls include gravity or mechanically stabilized earth (MSE) types of walls with an approximate 4 foot exposure height. Figure 11 shows a gabion wall that is used along the trail near eastbound Moffett Creek Bridge. With the estimated length of retaining walls approximately 13,200 feet, cut and fill walls are necessary throughout this 9,100 foot long trail.

Option 3B‐ Overland Trail with Saddle Cut: Option 3B connects west to east with a similar overland trail, but reduces some trail switchbacks and trail length, by excavating a portion of the area known as the “saddle”. The cut depth at the saddle is approximately 50 feet.

As in Option 3A, the steep terrain also requires retaining walls as the trail traverses switchbacks. Generally, feasible retaining walls include gravity or mechanically stabilized earth (MSE) types of walls with an approximate 4 foot exposure height. With the estimated length of retaining walls approximately 8,750 feet, cut and fill walls are necessary throughout this nearly 7,500 foot long trail.

5.4 ALIGNMENT 4 – I‐84 ADJACENT OPTION

5.4.1 OVERVIEW

Alignment 4 attempts to take fullest advantage of the existing I‐84 right‐of‐way. It is a roadside option with some earthwork. A preliminary look at available right‐of‐way next to the roadway shoulder indicates enough space exists for a path and barrier. The trail begins near the parking lot and swings wide with a large radius curve, then nearly parallels the roadway ramp down to I‐84. The 75’ vertical difference between the trail beginning and the bottom of the slope is traversed using a series of 8% grade runs and 2% grade resting areas. The portion of the alignment along the I‐84 shoulder will be separated by a crash‐worthy barrier with a pedestrian railing attached. At the east end of the point, the trail curves southward with a series of large radius switchbacks to tie into the existing access road. The grade difference is similarly achieved through a series of 8% runs with resting areas. Fill slopes of 2H:1V and railing were used in this alignment option at both the beginning and end alignment curves. Similar to the Alignment 2 options, the rockfall hazard along Alignment 4 suggests mitigation measures are necessary to protect trail users.

5.4.2 STRUCTURAL DISCUSSION

Option 4 connects west to east with a trail, whose grade is essentially off the shoulder of eastbound I‐

84.

Rockfall Mitigation:

Similar to Options 2A and 2B, utilizing the original HCRH alignment for the trail, the proximity of the eroding rock face to the trail necessitates the consideration of rockfall mitigation. However, in the vicinity of the chasm, the rock face moves away from the trail alignment. It is reasonable to consider rockfall mitigation unnecessary in the vicinity of the chasm.

The rock face area that would likely need rockfall mitigation in the form of mesh and cable netting was estimated from rough concept level ground contours and quantified in the cost estimate. Netting is estimated to be approximately 326,600 square feet.

In order to estimate rock shelter lengths, satellite photos and available contour maps were used. The western approach rock shelter is estimated to extend approximately 560 feet in length. The eastern approach rock shelter is estimated to extend approximately 600 feet in length.

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